Support build with cloudbuild for gcr

[sanposhiho]

part of #19

guide: https://github.com/kubernetes/test-infra/blob/master/config/jobs/image-pushing/README.md

For the most part, it is almost the same as the example in the guide.

[sanposhiho]

/kind feature

[sanposhiho]

image pushing job will be have been created in kubernetes/test-infra#23567

[sanposhiho]

/label tide/merge-method-squash

[Huang-Wei]

Is this conventional? I noticed some repos used gcr.io/k8s-staging-test-infra/gcb-docker-gcloud.

[sanposhiho]

If your build process is driven by a Makefile or similar, you can use GCB to invoke that. We provide the gcr.io/k8s-testimages/gcb-docker-gcloud image, which contains components that are likely to be useful for your builds.
https://github.com/kubernetes/test-infra/blob/master/config/jobs/image-pushing/README.md#makefile-build-example

So, gcr.io/k8s-staging-test-infra/gcb-docker-gcloud image is for Makefile build, maybe...?

Because simple build example uses gcr.io/cloud-builders/docker (and I followed it.)
https://github.com/kubernetes/test-infra/blob/master/config/jobs/image-pushing/README.md#simple-build-example

[Huang-Wei]

/lgtm
/approve

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